Conventional carbon steels and low-alloy steels are used as structural steels and simpler tool steels. As regards structure, these steels can be subdivided into ferritic-pearlitic steels and martensitic steels. The former steels are weldable but have relatively low mechanical strenght. Using martensitic steels it is possible to achieve considerably greater strenght, but this is normally obtained at the expense of toughness and weldability.
A number of new grades of steel have been developed in order to improve the weldability of the martensitic structural steels and at the same time retain their good strength properties, these being characterised by having a low carbon content whilst at the same time they have as alloy constituents mainly manganese and chromium and generally also some grain refining agents such as niobium, vanadium or titanium. Representative steels appertaining to this category described for example in Swedish patent specification No. 303 885 and in British patent specification Nos. 1 340 744 and 1 353 762. With these and other steels of similar composition important improvements in properties have been achieved in many respects. However the strength requirements imposed on structural steels designed for extremely demanding applications have gradually been increased to values which cannot be complied with by these earlier proposed steels. In particular it has proved difficult with these steels to achieve the desirable impact toughness at low temperatures whilst maintaining good tensile strength. Hardenability too is limited, which restricts the use of the steels for products having large dimensions. Such a product is anchor chain for off-shore oil drilling platforms.